In a method known, for example, from German Patent No. 44 20 962, for manufacturing self-supporting MEMS structures (MEMS=“micro electromechanical structures”) by combining anisotropic and isotropic etching processes, trenches or deep structures having perpendicular side walls are initially anisotropically etched into a silicon substrate using a reactive plasma. After achieving the intended etch depth and subsequent to a longer lasting passivation step for removing a PTFE-like film, a longer lasting etching step is carried out in which the etch bottom of the trench is initially cleared of a formed PTFE polymer via sputtering, followed by an isotropic underetching of the self-supporting MEMS structures to be created. Due to the previously applied PTFE-like film, the perpendicular side walls of the trenches remain protected from an etch attack during this isotropic underetching. According to German Patent No. 44 20 962, the isotropic underetching of the created MEMS structures proceeds in the silicon as the substrate material in a purely time-controlled manner.
A fluorohydrocarbon having an as low as possible fluorine/carbon ratio of preferably 2:1, particularly preferably less than 2:1, for example 1.5:1, is used for depositing the PTFE-like film; fluorohydrocarbons such as C4F6, C5F8, C4F8, or C3F6 are particularly suitable for this purpose. A per se isotropically etching and fluorine-delivering process gas, such as SF6, is preferably used for etching silicon.
German Published Patent Application No. 198 47 455 proposes vertically limiting the isotropic underetching during an isotropic underetching over buried oxide layers. Using an intermediate oxide, which separates the MEMS structure from a sacrificial layer made of silicon, an etch attack on the MEMS structures is prevented according to this publication. In addition, it is described there that, instead of using fluorine radicals from a plasma discharge, the isotropic underetching may also be performed using spontaneously and plasmaless silicon-etching fluorine compounds, such as XeF2, ClF3, or BrF3. After adsorption and chemisorption on a silicon surface, these compounds spontaneously separate fluorine radicals, which results in an isotropic removal of the etch-material of the silicon due to the formation of silicon tetrafluoride. At the same time, a very high selectivity is established with respect to non-silicon materials, such as PTFE-like or otherwise composed passivation layers. Photo resist in particular is attacked by ClF3 in a practically immeasurable manner, so that parts of a silicon wafer not to be etched are able to be passivated in a particularly simple manner while at the same time very thin passivation layers are already sufficient to ensure complete protection from an etch attack. The “native” silicon oxide, which as a rule is already present on silicon surfaces, is in many cases capable of withstanding a chlorine trifluoride attack for minutes without the silicon situated below being etched.
A further aspect in the use of ClF3, and to a limited degree also BrF3, is the inherently low reactivity of these high-oxidizing fluorine compounds vis-à-vis silicon which results in the fact that etching, carried out with these etchants, proceeds via an additional parameter area in a reaction-controlled manner and is not limited by a substance transport. Insofar, large undercut widths may also be achieved without a decrease in the etch rate due to an increasing aspect ratio of the undercut channels. By using these gases, ClF3 in particular, it is possible for example that the lateral expansion of the undercut channels constitutes a multiple, 100 to 1,000 for example, of its vertical expansion without adverse effects on the etching process.
Finally, isotropic etching using these gases takes place entirely without an ion effect which is a great advantage with respect to the intended isotropy and selectivity vis-à-vis non-silicon materials.
Overall, ClF3 is in many respects an ideal gas for selective removal of silicon or porous silicon within the scope of sacrificial layer technology, so that self-supporting diaphragm structures may be implemented very easily with minor limitations to the freedom of design. Even a single etch opening is sufficient in many cases to achieve a complete underetching of a diaphragm area on porous silicon, for example.
However, the use of ClF3, and with limitation also BrF3, has the disadvantage that it easily spreads in an uncontrolled manner over micro-channel structures or even nano-channel structures, so that there is a significant risk of it creeping extremely quickly behind applied passivation layers, side wall passivation layers according to German Patent No. 42 41 045 for example, via micro-cracks or nano-cracks in the border surface area to the silicon.
It has been found that a PTFE-like plasma passivation layer, created from C4F8 or C3F6, has a sufficient number of micro-channels or nano-channels in the border surface area to the silicon situated below, that, starting from a single inlet opening, for example on the etch bottom of the initially created trench, a large area of the entire MEMS structure is exposed to a ClF3 attack despite the applied PTFE-like passivation. A typical error image in this context is that structures, in which the etch bottom was previously cleared, supported by ions, of a passivating PTFE polymer according to German Patent No. 42 41 045, are massively etched on the entire, inherently passivated side wall surface, while structures, in which the etch bottom carries a rest of a PTFE-like polymer of only a few nanometers, do not show any sign of an etch attack, even after a long time. The cause of this problem is the explained micro-channels or nano-channels in the border surface area between the PTFE passivation, which adheres poorly to silicon, and the silicon surface which allow the etching gas ClF3 access to the silicon in undesirable areas.